A polyester, particularly polyethylene terephthalate, is widely used in fields of various molded articles such as a fiber, a film and a resin. The polyethylene terephthalate is generally produced by a method which comprises reacting dimethyl terephthalate or terephthalic acid with ethylene glycol in the presence of a catalyst. At that time, to enable the polyethylene terephthalate to satisfy properties required for a specific application, functional additives such as a stabilizer, a colorant and an antistatic agent are used.
In recent years, disposal of a polyethylene terephthalate molded article, particularly a post-consumer polyethylene terephthalate bottle (PET bottle) is now a social problem in that it pollutes the environment. Accordingly, its collection and recycling are under way.
As one method therefor, a method comprising the steps of collecting a used polyethylene terephthalate molded article, crushing the article into chips or flakes, depolymerizing them with ethylene glycol, purifying the resulting product so as to obtain high-purity bis(2-hydroxyethyl)terephthalate, and then polymerizing the bis(2-hydroxyethyl)terephthalate so as to obtain polyethylene terephthalate is under study.
Based on an idea that in order to obtain a polyester of high quality, a raw material of the polyester must also be of high quality, the present inventor has studied the method comprising the steps of depolymerizing chips or flakes of collected polyethylene terephthalate with ethylene glycol and purifying the resulting product so as to obtain high-purity bis(2-hydroxyethyl)terephthalate. Then, as a finding obtained from the study, the present inventor has proposed a method in which a bis(2-hydroxyethyl)terephthalate solution composition containing ethylene glycol, bis(2-hydroxyethyl)terephthalate and cations and anions as impurities as a solution resulting from the depolymerization reaction is brought into contact with a cation exchanger and an anion exchanger so as to give high-purity bis(2-hydroxyethyl)terephthalate (for example, refer to patent document 1).
Further, as a result of further study on the method, the present inventor has found that since hydrogen ions are liberated by the foregoing cation exchange treatment, a resulting treated solution becomes strongly acidic and bis(2-hydroxyethyl)terephthalate therefore becomes liable to cause an ester interchange reaction with diethylene glycol or a hydrolysis reaction, so that a reduction in yield of the target bis(2-hydroxyethyl)terephthalate or a reduction in purity of the bis(2-hydroxyethyl)terephthalate caused by inclusion of the above reaction products in the bis(2-hydroxyethyl)terephthalate are liable to occur disadvantageously.
(Patent Document 1)
International Publication No. 01/10812 Brochure
An object of the present invention is to provide a method for deionizing a decomposition produced solution resulting from decomposition of a polyester by ethylene glycol, particularly a decomposition produced solution containing, as a main component, bis(2-hydroxyethyl)terephthalate resulting from decomposition of a recovered polyethylene terephthalate by ethylene glycol.
Another object of the present invention is to provide a deionization method in which an ester interchange reaction and hydrolysis reaction of a decomposition product resulting from decomposition of a polyester by ethylene glycol, i.e., bis(2-hydroxyethyl)terephthalate, along with cation removing treatment are suppressed.
Still another object of the present invention is to provide a deionization method in which a reduction in yield and a reduction in purity which are caused by an ester interchange reaction or hydrolysis reaction caused by a decomposition product resulting from decomposition of a polyester by ethylene glycol hardly occur.
Other objects and advantages of the present invention will be apparent from the following description.